Cerebral Knievel:ZAZ: In my day dissecting frogs was the pinnacle of high school biology. A generation later students could make glowing frogs. In 2030 school children will be designing and growing eight foot tall giant ants for the stage version of Them. (Yes, feet, the U.S. will stand alone as metric deniers.)

in my day... the frog was the lead up to the pig fetus... which some of us jackasses in class used the small intestine as a jump rope... I wanna say this was about... 1991-2?

We dissected about 20 different animals in my 7th or 8th grade science class. Birds, frogs, fish, squid, cats... I can't even remember half of them.

Animatronik:An exoskeleton also makes growing in size a lot less convenient.

We don't have to periodically shed our bones to grow new ones. It tends to give smaller critters an advantage because they get more support from less exoskeleton.

Well, arthropods have reached some fairly impressive sizes in the past, and then there's these guys here:

The whole exoskeleton thing doesn't seem to be that big a burden, as long as the bug has a safe place to molt. What I wonder about is why no arthropod has evolved a way to get around the breathing thing.

Guess I know what I'll do when home DNA engineering kits come on the market... ;)

/like the mites who live in your eyebrows.

Actually, that never bothered me. 90% of the cells i our bodies aren't human, after all, so a few mites extra doesn't really matter. If you're looking for gross-out factor, my I suggest going to youtube and typing "black white head nose 3"? ;)

Fano:ZAZ: In my day dissecting frogs was the pinnacle of high school biology. A generation later students could make glowing frogs. In 2030 school children will be designing and growing eight foot tall giant ants for the stage version of Them. (Yes, feet, the U.S. will stand alone as metric deniers.)

IlGreven:thatboyoverthere: To be specific the cube-square law in relation to their trachea. They get too big and the trachea can not take in enough oxygen to keep up cellular respiration./Thank God.

Basically, 1: Their legs would not be strong enough to hold their weight. 2: Those with wings certainly would not be able to fly, and 3: their lungs would not be big enough to service their now giant bodies.

If the only criteria involves the hard-shell exoskeleton, then most of these statements about big bugs being impossible are misguided.

If big-insect lung and circulatory systems would be needed, that can be added through evolution or genetic engineering. It's not as if all other animals with complex respiratory and circulatory systems just sprung forth with it one day fully developed*, and the same can be said for evolved huge insects.

Exoskeleton weight is not really a problem. The solution is basically something we humans do already with surfboards: produce a hardened but thin exterior shell over the top of a rigid air-foamed core.

way southOf course, the question is why didn't insects just evolve a way around it. Moving past tracheal passages and developing lungs or some other mechanism to get more oxygen from the air.

AFAIK, tracheal passages are not a precursor to lungs. They are evolutionarily unrelated structures that merely provide a similar function. There is zero reason for bugs to abandon a perfectly good respiratory system in favor of a long and costly development process that may not even work out.

That is,

1) Bugs are everywhere and appear to be doing just fine. Any lung developments would need to provide a very significant advantage in order for them to be competitive with other bugs. Size is not a significant advantage, as (for example) ants are fairly primitive yet still manage to outnumber us by mass.

2) Tracheae work just fine. The bugs would need to be much larger for there to be an advantage to lungs. As noted above, size does not really limit propagation of a species, merely individuals. This one's a bit of a catch22.

3) Replacing tracheae with lungs would be significantly more difficult (i.e. nearly impossible) than just having evolved lungs in the first place. You need to remove something that works and replace it with something that won't work as well for many hundreds of thousands of years (or ever). You might as well ask why humans haven't evolved beyond our basal ganglia.

4) I imagine there are some engineering problems as well. Can bugs arbitrarily increase their volume to accomodate large amounts of inhaled air? Adapting to full lungs might make their exoskeletons useless.

falkone32:3) Replacing tracheae with lungs would be significantly more difficult (i.e. nearly impossible) than just having evolved lungs in the first place. You need to remove something that works and replace it with something that won't work as well for many hundreds of thousands of years (or ever). You might as well ask why humans haven't evolved beyond our basal ganglia.

You wouldn't have to replace the trachea. Instead, let them form a loop, add some valves and a pump (or just let them contract), and you have a continuous flow of air. Also, many spiders have lungs *and* trachea.

That's the term for the standard spider lung, but there is no homologous structure in insects. Or if there is it isn't used for anything analagous.

If you're interested in what goes on inside a spider, read Biology of Spiders by Rainer Foelix. But only if looking at slide preparations of bug guts is your thing. It's not a "oh look how cute they are" kind of book (despite the cover).